The entire disclosure of Japanese Patent Application No. 2006-043527, filed Feb. 21, 2006 is expressly incorporated by reference herein.
1. Technical Field
The present invention relates to the field of an image input device.
2. Related Art
An example of a known image input device is that in which respective light beams from a light source of LEDs corresponding to each color of R (red), G (green), and B (blue) are irradiated on a document and light beams reflected from the document are photoelectrically converted by an image sensor, thereby electrically reading the document (see JP-A-11-055471, for example). Specifically, JP-A-11-055471 discloses a handheld-type image input device. The image input device is provided with an encoder detecting a movement amount of the document, and the encoder outputs an encoder pulse when detecting a predetermined movement amount. In the image input device, the LEDs corresponding to each color of R, G, and B are sequentially lighted and extinguished at the time of receiving the encoder pulse to read out data corresponding to one scanning line, and then the LEDs are extinguished until receiving a subsequent encoder pulse.
As the image input device using an image sensor, there is known a device in which a carriage mounted with an image sensor and LEDs of three colors R, G, and B is driven by a DC motor connected to an encoder so that feedback control is performed therebetween. In such a type of image input device, when the carriage is moved by an amount corresponding to one scanning line (in a sub-scanning direction) by the DC motor, the encoder outputs a signal. Then, the image input device outputs a shift signal SH in synchronization with receiving the signal from the encoder and starts an operation of reading reflected light beams of a first color corresponding to one sub-scanning line in response to the shift signal. Operations of reading reflected light beams of the remaining two colors are performed in response to a subsequent shift signal that is output with a predetermined time lapse after the previous shift signal used for the first color, by using a timer.
Here, an operation of the image sensor in the image input device in which data is read in accordance with the signal from the encoder will be described with reference to FIG. 4.
In the image input device, a shift signal SH a20 is output in synchronization with receiving an edge signal a10 from an encoder so as to light an LED of a first color (for example, an R color) of three colors R, G, and B in response to the shift signal a20 (A3) and extinguish the first-color LED after a predetermined storage time T1. The image sensor stores electric charge in its light-receiving element during the lighting period of the first-color LED. Then, the image sensor outputs the stored electric charge d1 of the first color through a transmission path in synchronization with receiving a subsequent shift signal SH a21. Next, in the image input device, an LED of a second color (for example, a G color) is lighted in synchronization with receiving the subsequent shift signal SH a21 for the predetermined storage time T1 (A4). Similarly, as in the case of the first color, the image sensor stores electric charge in the light-receiving element during the lighting period of the second-color LED and outputs the stored electric charge d2 of the second color through the transmission path in synchronization with receiving a subsequent shift signal SH a22. Next, in the image input device, an LED of a third color (for example, a B color) is lighted in synchronization with receiving the subsequent shift signal SH a22 for the predetermined storage time T1 (A5). The image sensor stores electric charge in the light-receiving element during the lighting period of the third-color LED. Thereafter, the image sensor outputs the stored electric charge d3 of the third color through the transmission path in synchronization with a shift signal SH a200 that is output in synchronization with receiving an edge signal corresponding to a subsequent line. With these operations, data corresponding to one line is read out.
However, the image input device disclosed in JP-A-11-055471 or the motor-driven carriage-type image input device has the following problems.
When the data corresponding to each line are sequentially read in accordance with the signal from the encoder, the last color in one line may have a relatively greater output noise level than those of the remaining two colors. Specifically, the speed of the sub-scanning operation may not be maintained at a constant value in the case of the hand-held type image input device disclosed in JP-A-11-055471. In the case of the motor-driven carriage-type image input device, the speed of the sub-scanning operation may vary with the speed of the motor. Therefore, there may be an increase in the period (T10 in FIG. 4) between the time of extinguishing the third-color LED and the time of receiving the signal from the encoder. Moreover, the image sensor may receive noise such as light or heat from the outside even after the extinguishing of the LED, so that electric charge resulting from the noise are stored in the image sensor. As the period between the time of extinguishing the third-color LED and the time of receiving the signal from the encoder increases, the amount of stored electric charge corresponding to the third-color LED increases compared with those of the remaining two colors. As a result, there is an increase in the output noise level of the last color in the image sensor (for example, “d3” in FIG. 4, corresponding to a blue color's output noise), which thereby deteriorates the quality of the images.